No prior art search was conducted on the subject matter of this specification in the U.S. Patent and Trademark Office or any other search facility. We are unaware of any prior art attachment method for attaching a metal shaft to a ceramic shaft or product produced thereby which is relevant to the method and product disclosed in this specification other than the information which is set out hereinbelow.
Parts formed from high temperature resistant ceramics are usually thought of as replacements for high temperature parts in turbine and piston engines. Presently, the parts for such turbines and piston engines are made of very expensive metal alloys which have operating temperature limits in a range of 1800.degree.-2000.degree. F. Such temperature limits presently limit the thermal efficiency of turbine and piston engines. The thermal efficiency of such turbine and piston engines can be improved if ceramic parts are substituted for the metal alloys as the ceramic parts could have operational temperature limits in a range of 2300.degree.-2500.degree. F. or higher. Such an increase in operational temperature, of course, provides for greater thermodynamic efficiency as is well known in the art.
Unfortunately, replacing metal alloy parts with ceramic parts generally requires a ceramic to metal joint at some point. These joints are difficult to form because ceramics have very low thermal expansion rates as compared to the thermal expansion rates of the metal alloys now being used. Thermal expansion rate is a measure of how fast a sample of material expands or grows as it is heated. The coefficient of thermal expansion is generally noted as alpha (.alpha.) and its units are inches per inch per degree farenheit. Generally, a one inch long steel bar will expand about ten millionths of an inch per one degree farenheit increase in temperature (alpha=10.times.10.sup.-6 in/in/.degree. F.). Silicon nitride ceramics generally expand at about one-fifth of the rate for steel (alpha=1-2.times.10.sup.-6 in/in/.degree. F.).
If a ceramic/metal attachment joint is heated or cooled from its assembly temperature, the steel half of the joint changes shape and size five times faster than the ceramic half of the joint. The steel half of the joint attempts to drag the ceramic half of the joint with it either through some interlocking feature such as ceramic over steel shaft joint, or a locking caused by frictional forces. Unfortunately ceramic to metal joints exhibit very high coefficients of friction. The growth of ceramic and steel parts due to heating is unstoppable. Any externally applied forces used to try to maintain original shapes during heating of parts which are subject to such growth merely cause the piece being restrained to develop whatever forces are required to overcome the restraint. Hence, if the ceramic half of the ceramic/metal joint tries to restrain a more rapidly expanding metal, huge forces rapidly develop in the ceramic/metal joint which result in rapid and certain failures to the ceramic portion of the joint.
In the past, we have attached ceramic rotors with a mounting system that used curvic gear teeth between the ceramic part and the metal part to hold the parts together. The frictional forces generated between the teeth during heating of the ceramic and metal joined structure caused rapid failure of the ceramic part of the joint. A 100.degree. F. increase in temperature above assembly temperature of the ceramic/metal joint was enough to break the ceramic half of the joint due to the forces applied thereon by the growing metal half of the joint. The teeth of the metal curvic gear had to be plated with pure gold in order to cause a slipping condition between the teeth rather than a lockup between the teeth of the ceramic half and the metal half of the joined structure. Gold plating in such a manner had the ability to provide operation of such a joined structure at a temperature in a range of 1400.degree.-1800.degree. F., but the lubricating ability of the gold plate was only good for 8-10 thermal cycles on the joint. After this number of thermal cycles, the joint would fail. Additionally, the curvic teeth are ground by special machinery and very few people are skilled in the manufacture of such teeth. The cost of grinding the teeth was several hundred dollars per set of teeth, and mounting one rotor required four such sets of teeth.
The method of attaching a metal shaft to a ceramic shaft set forth in this specification is one designed to reduce the severity of the thermal gradients set forth in the joint by placing in an area of the turbine bearing compartment where the temperature during operation generally would not exceed 400.degree. F. The unique method of attaching a metal shaft to a ceramic shaft and product produced thereby will be discussed in greater detail hereinbelow, but such a method is economical to carry out and very reliable in joining such structures.